Wafer chuck with aerodynamic design for turbulence reduction

ABSTRACT

A rotatable wafer chuck includes chuck arms and wafer holders that are aerodynamically shaped to reduce turbulence during rotation. A wafer holder may include a friction support and an independently rotatable vertical alignment member and clamping member that is shaped to reduce drag. The shape reduces turbulence during edge bevel etching to improve the uniformity of the edge exclusion and during high-speed rotation to improve particle performance.

FIELD OF THE INVENTION

The present invention relates generally to methods and apparatus forsemiconductor processing and more particularly to a wafer chuck andmethods for rotating the wafer in a chuck. It is particularly useful forpost copper electroplating processing in damascene and dual damasceneintegrated circuit fabrication methods.

BACKGROUND OF THE INVENTION

This invention relates to technology for rotating a semiconductor waferduring post copper electroplating processing in a post electrofillmodule (PEM). More particularly, the invention pertains to wafer chucksused in PEMs to align, rotate and clamp the semiconductor wafer. Themodules typically perform a metal etch removing the edge bevel copper,rinse the wafer, and dry the wafer. The PEMs are typically a part of anintegrated electrofill modules that includes metal deposition, etching,and any other pre and post treatment.

During integrated circuit fabrication, conductive metal is needed on theactive circuit region of the wafer, i.e., the main interior region onthe front side, but is undesirable elsewhere. In a typical copperDamascene process, the formation of the desired conductive routesgenerally begins with a thin physical vapor deposition (PVD) of themetal, followed by a thicker electrofill layer (which is formed byelectroplating). The PVD process is typically sputtering. In order tomaximize the size of the wafer's useable area (sometimes referred toherein as the “active surface region”) and thereby maximize the numberof integrated circuits produced per wafer), the electrofilled metal mustbe deposited to very near the edge of the semiconductor wafer. Thus, itis necessary to allow physical vapor deposition of the metal over theentire front side of the wafer.

As a byproduct of this process step, PVD metal typically coats the frontedge area outside the active circuit region, as well as the side edge,and to some degree, the backside. Electrofill of the metal is mucheasier to control, since the electroplating apparatus can be designed toexclude the electroplating solution from undesired areas such as theedge and backside of the wafer. One example of plating apparatus thatconstrains electroplating solution to the wafer active surface is theSABRE™ clamshell electroplating apparatus available from NovellusSystems, Inc. of San Jose, Calif. and described in U.S. Pat. No.6,156,167, “Clamshell Apparatus For Electrochemically TreatingSemiconductor Wafers,” issued to E. Patton et al. on Dec. 5, 2000, whichis herein incorporated by reference in its entirety for all purposes.

The PVD metal remaining on the wafer edge after electrofill isundesirable for various reasons. One reason is that PVD metal layers arethin and tend to flake off during subsequent handling, thus generatingundesirable particles. This can be understood as follows. At the frontside edge of the wafer, the wafer surface is beveled. Here the PVDlayers are not only thin, but also unevenly deposited. Thus, they do notadhere well. Adhesion of subsequent dielectric layers onto such thinmetal is also poor, thus introducing the possibility of even moreparticle generation. By contrast the PVD metal on the active interiorregion of the wafer is simply covered with thick, even electrofill metaland planarized by CMP down to the dielectric. This flat surface, whichis mostly dielectric, is then covered with a barrier layer substancesuch as SiN that both adheres well to the dielectric and aids in theadhesion of subsequent layers.

Wafer chucks have been designed that can hold the semiconductor waferduring the metal etch. The system may align the wafer on chuck forrotation. Conventionally, such alignment is done by placing the wafer onthe chuck that has a self-aligning capability or in a separate alignmentmodule and then transporting it the chuck. The wafer chuck does notcontact the wafer edges during the actual etching of unwanted metal fromthose regions. Otherwise, the viscous etchant would not be able to flowover the side edge of the wafer unimpeded. During rinsing and drying,the wafer may rotate at high-speeds, at which time the wafer ispreferably constrained to the wafer chuck. Such a wafer chuck isdescribed in U.S. Pat. No. 6,537,416 issued to Mayer et al. on Mar. 25,2003, U.S. Pat. No. 6,967,174 issued to Mayer et al. on Nov. 22, 2005,both titled “Wafer Chuck For Use In Edge Bevel Removal of Copper FromSilicon Wafers” and are incorporated by reference herein in its entirelyfor all purposes. The wafer chuck is also described in U.S. patentapplication Ser. No. 11/248,874, filed Oct. 11, 2005, titled “Edge BevelRemoval of Copper From Silicon Wafers,” which is incorporated byreference herein in its entirely for all purposes.

However, some problems have been observed with the process using thewafer chuck. The edge exclusion (EE), the portion of the wafer etchedduring EBR, has been observed to vary at different locations of thewafer. Irregularity of the etchant stream may cause this variation. Attimes, this EE variation manifests as a wave in the copper around theedge, caused by incomplete removal in some areas. Another problem isparticles transfer to the wafer by the mechanical action of the tool.Tool availability may be reduced by adders such as particles, droplets,streaks, or any other contamination that can be entrained andtransported to the wafer surface because the test is re-run until thecleanliness reduces to some prescribed level.

Therefore, an improved wafer chuck design is desirable.

SUMMARY OF THE INVENTION

The present invention provides wafer chucks designed to solve theabove-described problems, among others. An improved rotatable waferchuck includes chuck arms and wafer holders that are aerodynamicallyshaped to reduce turbulence during rotation. A wafer holder may includea friction support and an independently rotatable vertical alignmentmember and clamping member that is shaped to reduce drag. The shapereduces turbulence during edge bevel etching to improve the uniformityof the edge exclusion and during high-speed rotation to improve particleperformance.

While the wafer chuck is described in the context of post electrofillmodules, similar designs may be employed with other types ofsemiconductor processing modules. It will be apparent to those ofordinary skill in the art that various specifications, alignment andclamping timings/rotational speeds can be modified to adapt the waferchuck of the present invention to other semiconductor wafer processes.

One aspect of the invention provides for a wafer chuck with a number ofchuck arms extending radially outwards from the center of the chuck.Each chuck arm ends in a wafer holder, and each wafer holder has a frontend and a back end along the direction of the rotation of the chuck.Either end or both may be a turbulence reducing structure having aaerodynamic shape. The wafer holder may include an alignment member anda clamping member. The aerodynamically shaped portion may be theclamping member.

The aerodynamic shape may have sharp or rounded edges. The shape may be,for example, an elliptic paraboloid or a cone, with a portion or aquadrant removed, creating a shelf. A wafer on the chuck would occupythe shelf, with some clearance outward and below the wafer. The shapemay also be a prismatoid having three or more sides, with also a portionor a quadrant removed. A prismatoid is a polyhedron where all thevertices lie in one of two parallel planes. For example, a pyramid hasone vertex in one plane and 3 or more vertices in another plane. A wedgeis also a prismatoid having two vertices in one plane and four inanother. The shape may further include one or more curved lines and oneor more curved sides.

The wafer sits on friction supports on the chuck arms. When the chuck isat low rotational speed (below edge bevel reduction (EBR) speed) or atrest, the alignment members are in an inward, alignment position. At theinward position, the alignment pins make a slightly over sized pocket(for 300 mm wafers, just 0.06 mm larger) and may or may not contact thewafer edge. Above a certain rotational speed, the alignment members moveaway from the edge of the wafer, thus allowing the EBR etchant to flowover the edge of the wafer unimpeded. At these speeds, the clampingmembers are in an outward, non-clamping position. At still higherrotational speeds (e.g., drying speeds), the clamping members moveinward to clamp the wafer by contacting its edge. The invention providesfor alignment and clamping members that are independently rotatable in aperpendicular direction to the wafer surface. The members may be camsand be designed so that they will automatically move inward or outwardabout pivot pins, based on the direction of the centripetal force on thecams.

The turbulence reducing structure having an aerodynamic shape preferablysubstantially shields the alignment member from direct airflow while thechuck is rotating. Using a low drag shape reduces the air turbulencecreated by the high-speed rotation of the chuck. The drag coefficientmay be about 0.5 or less, preferably about 0.2 to 0.3. The overall angleof the turbulence reducing structure may be about 30-90 degrees,preferably 30-60 degrees. The overall angle is the apex angle of atriangle containing the turbulence reducing structure with the apexpointing in the forward direction of rotation and the base perpendicularto the wafer. In other words, this overall angle would affect the dragcoefficient and length of the turbulence reducing structure—smallerangle, longer structure with lower drag; bigger angle, shorter structurewith bigger drag. In certain embodiments, the total length of thestructures may overlap less than about 25% of the wafer perimeter,preferably less than about 10% of the wafer perimeter.

Another aspect of the invention provides for a method of processing asemiconductor wafer in a wafer chuck by providing a wafer to the waferchuck and rotating the chuck thereby rotating the wafer. The wafer chuckhas a number of arms and wafer holders at the ends of the arms. Thewafer holder may include a turbulence reducing forward end, a turbulencereducing rear end, or turbulence reducing forward and rear ends. Themethod may further include etching metal from the wafer, cleaning thewafer, and drying the wafer while it rotates. The method may alsoinclude aligning the wafer and clamping the wafer while it rotates at ahigh speed during the drying step.

These and other features and advantages of the present invention will bedescribed in more detail below with reference to the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a semiconductor wafer showing the locationof the edge bevel region that is etched in accordance with thisinvention.

FIG. 2 is a process flow diagram illustrating relevant operationsemployed to form conductive copper lines a Damascene process in thecontext of this invention.

FIG. 3 is a block diagram illustrating a group of modules used to formcopper conductive lines on an integrated circuit.

FIG. 4 is a block diagram illustrating various elements of apost-electrofill module in accordance with one embodiment of thisinvention.

FIG. 5 is a process flow diagram illustrating a typical sequence ofoperations employed with a post-electrofill module in accordance with anembodiment of this invention.

FIG. 6A is a schematic illustration of a simplified wafer chuck with twochuck arms and no alignment and clamping members.

FIG. 6B is a top-view schematic illustration of a wafer chuck with threechuck arms, each with an alignment/clamping member at the end.

FIG. 7 is a graphic table showing drag coefficients for different shapedobjects.

FIGS. 8-11 are a close-up schematic illustration of various embodimentsof a wafer holder in accordance with the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In the following detailed description of the present invention, numerousspecific embodiments are set forth in order to provide a thoroughunderstanding of the invention. However, as will be apparent to thoseskilled in the art, the present invention may be practiced without thesespecific details or by using alternate elements or processes. In otherinstances well-known processes, procedures and components have not beendescribed in detail so as not to unnecessarily obscure aspects of thepresent invention.

INTRODUCTION

As indicated, aspects of this invention pertain a wafer chuck designthat improves uniformity of edge exclusion etched during EBR andparticle performance. To facilitate understanding the concepts of thisinvention, a schematic illustration of a semiconductor wafer is shown inFIG. 1. As shown, such semiconductor wafer has a top or “front” side 100and a “backside” 101. The wafer also has an interior “active circuitregion” 102 where integrated circuit devices with associated conductivemetal routes are formed. To make maximum use of expensive semiconductormaterial, this active circuit region should constitute a high fractionof the area on the front side 100 of the wafer. With a 200 mm wafer, theuseable active surface region extends to within at least 1.5 and 4 mm ofthe outer boundary of the wafer. With a 300 mm wafer, the useable regionmay extend to within about 1.3 and 3.75 mm of the outer boundary of thewafer. In some cases, the useable region may be within about 1.75 and 2of the outer boundary of the wafer. As shown, integrated circuit wafersalso include a “front edge” area 103, which is the region on the frontof the wafer that lies outside the active circuit region, a “side edge”area 104 (sometimes referred to herein as an “edge bevel region”) and a“back edge” area 105. The side edge lies in the area between the frontside and the backside, and the back edge is roughly the area near theouter boundary of the wafer on its backside, approximately analogous tothe front edge area.

A “post-electrofill module” (PEM) or “EBR module” as referred to in thisinvention is a module that is specifically designed to carry out theedge bevel removal (EBR) process, backside etch (BSE) process andancillary processes including pre-rinsing, rinsing, acid washing anddrying. An integrated-electrofill module as referred to in thisinvention is a module that carries out electrofill.

While details of certain embodiments may be found below in thisapplication, a short description of a typical Damascene process will nowbe provided to facilitate understanding the context of the presentinvention as shown in FIG. 2. The process begins with formation of linepaths 201 in a previously formed dielectric layer. These line paths maybe etched as trenches and vias in a blanket layer of dielectric such assilicon dioxide. They define conductive routes between various deviceson a semiconductor wafer. Because copper or other mobile conductivematerial provides the conductive paths of the semiconductor wafer, theunderlying silicon devices must be protected from metal ions (e.g.,copper) that might otherwise diffuse into the silicon. To accomplishthis, the process includes depositing a thin diffusion barrier layer 202before depositing the metal. Suitable materials for the diffusionbarrier layer include tantalum, tantalum nitride, tungsten, titanium,and titanium tungsten. In a typical embodiment, the barrier layer isformed by a PVD process such as sputtering.

The wafer is now nearly ready to have its line paths inlayed with theelectrofill copper. However, before electrofilling, a conductive surfacecoating must be applied. In the depicted process, this is accomplishedby depositing a copper seed layer on the barrier layer at 203. A PVDprocess such as sputtering may be employed to this end. A thicker layerof bulk copper is then deposited over the seed layer 204, typically byelectroplating using an electroplating solution. The copper is depositedto a thickness that completely fills the various line paths in thedielectric layer.

During the deposition of PVD copper in some unwanted areas cannot beavoided. This copper must be removed, and this is accomplished by theedge bevel removal (EBR) and/or backside etch (BSE) processes. With EBRat 205, a copper etchant is applied to the front edge of the wafer in athin stream. The etchant is preferably applied under viscous flowconditions so that it remains in a thin, viscous layer near the point onthe wafer where it is applied, and thus avoids splashing the interior ofthe wafer and removing wanted copper from the active circuit region. Itis important for the stream of etchant to apply evenly, otherwisevariations in edge exclusion would result. An uniform edge exclusioncreates the maximum active and useable surface area. Because the etchantis also generally applied with a radial velocity component, and becauseof the centripetal acceleration effects of the rotating wafer, the thinviscous layer flows outward, down over the side edge and a fewmillimeters onto the backside, thus accomplishing removal of the PVDcopper from all three of these areas. After EBR, the electroplatedcopper is planarized, generally by chemical-mechanical polishing (CMP)down to the dielectric at 206 in preparation for further processing(207), generally the addition of subsequent dielectric and metalizationlayers.

FIG. 3 depicts an electrofill system 307 in which the invention mayreside. The specific system includes three separate electrofill modules309, 311 and 313. System 307 also includes three separate postelectrofill modules (PEMs) 315, 317 and 319. Each of these may beemployed to perform each of the following functions: edge bevel removal,backside etching, and acid cleaning of wafers after they have beenelectrofilled by one of modules 309, 311, and 313. System 307 alsoincludes a chemical dilution module 321 and a central electrofill bath323. This is a tank that holds the chemical solution used as theelectroplating bath in the electrofill modules. System 307 also includesa dosing system 333 that stores and delivers chemical additives for theplating bath. A chemical dilution module 321 stores and mixes chemicalsto be used as the etchant in the post electrofill modules. A filtrationand pumping unit 337 filters the plating solution for central bath 323and pumps it to the electrofill modules. Finally, an electronics unit339 provides the electronic and interface controls required to operatesystem 307. Unit 339 may also provide a power supply for the system.

In operation, a robot including a robot arm 325 selects wafers from awafer cassette such as a cassette 329A or a cassette 329B. Robot arm 325may attach to the wafer using a vacuum attachment or some otherattaching mechanism.

To ensure that the wafer is properly aligned on robot arm 325 forprecision delivery to an electrofill module, robot arm 325 transportsthe wafer to an aligner 331. In certain embodiments, aligner 331includes alignment arms against which robot arm 325 pushes the wafer.When the wafer is properly aligned against the alignment arms, the robotarm 325 moves to a preset position with respect to the alignment arms.In other embodiments, the aligner 331 determines the wafer center sothat the robot arm 325 picks up the wafer from the new position. It thenreattaches to the wafer and delivers it to one of the electrofillmodules such as electrofill module 309. There, the wafer iselectrofilled with copper metal. Electrofill module 309 employselectrolyte from a central bath 323.

After the electrofill operation completes, robot arm 325 removes thewafer from electrofill module 309 and transports it to one of thepost-electrofill modules such as module 317. There unwanted copper fromcertain locations on the wafer (namely the edge bevel region and thebackside) is etched away by an etchant solution provided by chemicaldilution module 321. The wafer is also cleaned, rinsed, and dried.

Preferably the wafer is precisely aligned within post electrofill module317 without making use of aligner 331. To this end, the post electrofillmodules may be provided with an wafer chuck capable of aligning thewafer. In an alternative embodiment, the wafer is separately alignedwithin aligner 331 after electrofill and prior to edge bevel removal inmodule 317.

After processing in post electrofill module 317 is complete, robot arm325 retrieves the wafer from the module and returns it to cassette 329A.From there the cassettes can be provided to other systems such as achemical mechanical polishing system for further processing.

The Post Electrofill Module (PEM) and Process

FIG. 4 schematically illustrates an embodiment of a post-electrofillmodule 420 suitable for use with this invention. As shown, module 420includes a chamber 422 in which a semiconductor wafer 424 rotates. Wafer424 resides on a wafer chuck 426 which imparts rotational motion towafer 424. Chamber 422 is outfitted with a drain and associated drainline 464. The drain allows the various liquid streams provided tochamber 422 to be removed for waste treatment.

A motor 428 controls the rotation of chuck 426. Motor 428 should be easyto control and should smoothly transition between various rotationalspeeds. It may reside within or without chamber 422. In someembodiments, to protect against damage from liquid etchant, motor 428resides outside of chamber 422 and is separated therefrom by a sealthrough which a rotating shaft 427 passes. Preferably, motor 428 canrapidly accelerate and decelerate (in a controlled fashion) chuck 426and wafer 425 at rotation rates between 0 and about 2000 rpm. The motorspeed and other operations should be controllable by a computer.

Chuck 426 may be of a design that holds wafer 424 in position duringvarious rotational speeds. It may also facilitate alignment of wafer 424for the etching process. Chamber 422 may be of any suitable design thatconfines the liquid etchant within its interior and allows delivery ofthe various fluids to wafer 424. It should be constructed of an etchantresistant material and include ports and nozzles for the various liquidand gaseous streams used during etching and cleaning.

Gaseous nitrogen or other non-reactive gas may be provided to postelectrofill module 420 from a gas source 430. Nitrogen from source 430is delivered to chamber 422 under the control of a valve 432. Thegaseous nitrogen is delivered into chamber 422 via a line and nozzle 434positioned to deliver the nitrogen in a downward approximately laminarflow. As the wafer 424 spins on the wafer chuck 426, a turbulenceextending to the wall of the chamber can entrain droplets of solution.The vortex may be suppressed by this downward flow of air, usuallynitrogen or other inert gas. However, to ensure suppression of thevortex, the air flow should preferably be highly laminar and have highvelocity, which requires extremely uniform exhaust and lots of nitrogen.The air velocity may be 45-90 feet per minute (ft/m), preferably 60-80ft/m. Protrusions from the wall, such as that for the etching fluiddispensing apparatus, also affects the downward air flow. Also, thedistance between the wafer and the wall should be minimized but shouldallow adequate clearance. The distance between the wafer and the modulewall may be 10-20 inches, preferably 14-15 inches. Lastly, the fasterthe chuck rotates, the more turbulence is created. Although SRD chucks(spin/rinse/dry only) may rotate at about 5000 rpm, at that speed theturbulence and entrainment are too great for the EBR chuck. In general,rotation speeds of about 0-2500 rpm, preferably 100-1500 rpm, and evenmore preferably 500-1300 rpm, are consistent with the present invention.

The next input is a source of deionized water 436. The deionized wateris delivered to chamber 422 under the control of a valve 438 and througha delivery line and nozzle 438. Note that line 438 directs deionizedwater onto the top of wafer 424. This enables rinsing of the wafer's topside. A preferred nozzle sprays fluid as a thin “fan” that spreads outover the inner three-quarters of the wafer diameter. The spray canimpact the wafer with a velocity in the same direction as the wafer isrotating, or opposite the direction of rotation, or even in bothdirections if the spray fan crosses the wafer center. Preferably, thespray is directed opposite to the direction of rotation to increaseconvective mixing.

A similar deionized water system provides a stream or fan of deionizedwater to the backside of wafer 424. This deionized water is providedfrom a source of deionized water 440, which may be the same as source436. A valve 442 controls the flow of deionized water onto the backsideof wafer 424 via a line and nozzle 444. The nozzle associated with 444may have the same design criteria as just mentioned for nozzle 438. Thegoal is to rinse etchant from the backside of wafer 424.

An acid rinse may be conducted on the front side of wafer 424. To thisend, a source of sulfuric acid 446 provides sulfuric acid to a deliveryline and nozzle 450. Other acids may be used as appropriate or combinedwith sulfuric acid. For example, hydrogen peroxide may be used.Preferably, this module includes a valve that controls the delivery ofsulfuric acid to module 420. The flow of sulfuric acid into chamber 422may be monitored by a mass flow meter 448. Note that in the depictedembodiment nozzle 450 is oriented to direct sulfuric acid onto thecenter of the front side of wafer 424. After the acid is delivered tothe center of the wafer it then spins out into the edge of the waferduring rotation. This solution is applied to remove residual copperoxide which remains after oxidizing (etching) the wafer and aids in theoverall cleaning of the wafer. Only a relatively small amount of acid istypically required (e.g., 0.5 to 2 milliliters/200 mm wafer). After itsapplication, the wafer's front side is rinsed with deionized waterthrough nozzle 438.

Liquid etchant used to remove copper or other unwanted metal fromportions of wafer 424 is provided from a source of liquid etchant 452 asshown. The etchant passes through a mass flow meter 454 and is deliveredto wafer 424 via a line and nozzle 456. Preferably, the etchant isdelivered precisely to the edge bevel region of wafer 424 to remove PVDcopper in that region only.

A second liquid etchant stream may be delivered to the backside of wafer424 in order to etch off any copper or other unwanted metal that mayhave been deposited on the backside of wafer 424. As shown, such etchantis delivered from an etchant source 458. Preferably, this is the samesource as 452. As shown, etchant from source 458 passes through a massflow meter 460 and through a nozzle 462, which directs it onto thebackside of wafer 424.

An example EBR process is illustrated in FIG. 5. The EBR process 500 canbe carried out by a post-electrofill module, such as module 420 of FIG.4, that is specifically designed to carry out the EBR process. Theprocess begins at 501, with a robot arm placing the wafer on the modulechuck for EBR processing. The wafer is typically aligned by a number ofsloped alignment members and placed on a set of frictional support pinsthat hold the wafer in place by static friction, even when the wafer islater rotated. After the robot arm retracts, deionized water is appliedto the front of the wafer and the wafer is spun at about 200-400 rpm inorder to pre-rinse the wafer of any particles and contaminants left overfrom previous steps 502. The deionized water is then turned off and thewafer is spun up to a speed of between about 350-500 rpm, which createsa uniformly thin layer of deionized water (wet-film stabilization) 503.This wet-film stabilization facilitates an even distribution of theetchant over the front side of the wafer. At this time, at the latest,any alignment pins or clamps that were used to precisely align the waferin the chuck are retracted from the edge of the wafer.

After wet-film stabilization 503, the core feature of the EBR, actualremoval of the edge bevel metal 504 is performed. The EBR etchant istypically applied to the surface of the wafer using a thin nozzle tube,which has a nozzle opening at or near its end. In a specific example, anEBR dispense arm is positioned over the wafer edge. Then EBR isperformed under the following conditions: a total of about 3 to 15milliliters etchant is delivered at a rate of about 0.2 to 3milliliters/second (more preferably about 0.3 to 0.4 milliliters/second)for a 300 millimeter wafer. In some embodiments, the etchant may bedispensed in two or more operations of different flow rates. In aparticular example, the 1 ml of the etchant is dispensed at 0.4 ml/secfor a first operation, then 10 ml of the etchant is dispensed at 0.3ml/sec for a second operation.

During EBR, some etchant may flow onto the backside of the wafer andetch it. An alternative embodiment for practicing the present inventionis to have the wafer facing upside down, and to apply the etchant to thebackside edge.

After the required amount of liquid etchant has been applied to the edgeof the wafer, deionized water is again applied to the front side of thewafer as a post-EBR rinse 505. This application of deionized water willgenerally continue through the subsequent operations of backside etchingand backside rinsing so as to protect the wafer from any extraneousbackside etchant spray and damage. While the deionized water is applied,the dispense arm moves the etchant nozzle away from the wafer.

At generally about the same time as commencement of step 505, thebackside of the wafer is pre-rinsed 506 with deionized water, which iswet-film stabilized 507 in much the same manner that the front side ofthe wafer was (e.g., the wafer rotation speed is held at about 350 to500 rpm). After the flow of deionized water to the wafer backside ends,a backside etch operation 508 is performed—generally with the sameetchant that was used for the EBR. In a specific embodiment, a thin jet(initially 0.02 to 0.04 inches in diameter) of liquid etchant is aimedat the center of the wafer backside. The etchant is delivered from atubular nozzle having a diameter of about 0.02 to 0.04 inches and alength of at least about 5 times the diameter. This etchant thendisperses over the entire backside of the wafer. The purpose of the BSEis to remove any residual copper that was formed on the backside of thewafer during formation of the seed layer of PVD copper.

The BSE etchant is typically applied using a spray nozzle. Despitegravity, surface tension generally keeps the etchant in contact with thebottom of the wafer long enough to carry out BSE. Since the chuck armscould interfere with the spraying of etchant on the backside of thewafer, the angle of the spray nozzle may be varied during BSE to ensurethorough application of the etchant. Because the wafer is generally heldup by support pins that impinge on the backside of the wafer, theprocess is generally carried out at two different speeds to ensure thatthe etchant flows adequately over the entire surface. For instance, thewafer may be rotated at about 350 rpm during part of the BSE and thenrotated at 500-700 rpm for the remainder of the BSE. The portions of thebackside blocked by the arms will differ at the two speeds, thusensuring complete coverage. Overall, the BSE process typically takes 1-4seconds and uses 1 to 5 cubic centimeters of the etchant described belowto reduce the concentration of copper on the backside to less than5×10⁻¹⁰ atoms per cm² of substrate.

After BSE, both sides of the wafer (or at least the backside of thewafer) are rinsed with deionized water to rinse any liquid etchant,particles and contaminants remaining from the BSE 509. Then the flow ofdeionized water to the front side ends and about 2 to 4 milliliters of adilute acid, generally less than about 15% by weight acid, is applied tothe front side of the wafer to remove residual metal oxide and removethe associated discoloration 511. In a specific embodiment, the acid isapplied at a rate of about 2 cc/sec. After the acid rinse, deionizedwater is once again applied to both sides of the wafer, or at least thefront side, to rinse the acid from the wafer. In a specific embodiment,the deionized water is applied for about 15-30 seconds at about 300-400milliliters/min. Finally the wafer can be spun and blow-dried, asdesired, on both sides with nitrogen 512. Generally, any drying step iscarried out at about 750-2000 rpm for about 10 to 60 seconds, andnecessitates a clamping for the wafer once it reaches about 750 rpm.After this processing in the PEM is completed, a robot arm picks up thewafer and puts it in a cassette.

The Wafer Chuck

As discussed above, the wafer chuck design must ensure that the EBRetchant flows in a steady viscous stream thereby etching the wafer atconstant radii. Uneven or non-uniform edge exclusion would result inless of the wafer being useable for active circuit. Additionally,turbulence induced by the high-speed rotation is believed to entraindroplets from the module chamber wall. The turbulence is a function ofchuck speed, which is much higher for 300 mm because the wafer holder isfurther away. These droplets may be sprayed back onto the clean wafersurface and produce measurable contamination and cause a particleexcursion. Such failure mode must be alleviated before the module may beused because particles can cause defects and reduce yield.

FIG. 6A is a highly simplified side-view of portions of a wafer chuck600 that is used to support and rotate a wafer in a PEM. FIG. 6B is ahighly simplified top view of the wafer chuck 600. Wafer chuck 600 isshown with three chuck arms 601. The arms may be evenly spaced aroundthe center of the chuck. At the end of each arm 601 is a wafer holderwhich includes an alignment member 611 and a clamping member 615. Thealignment members 611 are used to facilitate proper alignment of thewafer on chuck 500 as the wafer is delivered to chuck 600. Each chuckarm 601 has a support pin 602 that impinges on the bottom of the wafer(wafer not shown).

The wafer chuck 600 and the wafer rotate about a center of rotation in abase 603. Wafer chucks are also outfitted with clamping members 615 thathold the wafer in position during high rotation rates. Note that thealignment members 611 and clamping members may have a through hole 613,which allows etchant and other liquids delivered onto the wafer to flowoff the wafer and beyond the chuck, without becoming entrained on thealignment members or other elements of the chuck. Note also that whileFIGS. 6A and 6B are shown with only three chuck arms, other embodimentsmay employ more arms. In one specific embodiment, a wafer chuck of thisinvention includes six arms, three that have alignment members andanother three that have clamping members.

As discussed above, vortex turbulence caused by the wafer holderrotating is believed to entrain sidewall droplets and creates particles.The turbulence also affects the etchant stream from the EBR, which makesthe edge exclusion uneven. This effect is shown by the reduction of edgeexclusion variation at low rotational speeds and high levels ofvariation at high rotational speeds. One way to reduce the turbulence isto reduce the drag of the moving object. Drag may be affected by thevelocity, frontal area, angle of inclination, and the shape of theobject, among others. The shape of the object determines a dragcoefficient, which is usually determined experimentally. Aerodynamicshaping is known to reduce drag caused by the shape of the object. FIG.7 shows known drag coefficients (Cd), or form drag, for a number ofshapes. Empirical Cd are measured in calibrated wind tunnels for variousshapes. Cd's for various shapes may be found in many reference materialssuch as Fluid Dynamic Drag by Sighard Hoerner, which is hereinincorporated by reference for all purposes. For example, the dragcoefficient for a spherical cap is 0.38 (701); for a hemisphere, 0.42(703), for bean shape, 0.59 (7/5); for a diamond, 0.8 (707); and for anequilateral triangle, 0.5 (709) (Sighard Hoerner, Fluid Dynamic Drag,pp. 3-17). A wafer holder having these shapes would have lower dragcoefficients (less than 0.8) than one with a flat face facing thedirection of air flow (greater than 1.0). The present invention mayemploy any one or more of these shapes in the wafer holders foraerodynamic drag reduction.

Further streamlining the shape and adding an aerodynamic rear end mayreduce the drag coefficient to as low as 0.2 or below. The dragcoefficient must be balanced with the length of the structure, becausewhile longer structures may have less drag, such structures increase theweight of the wafer chuck, affecting the rotation mechanism. They alsoincrease the amount of the wafer perimeter covered by the structures,which as described above, should remain relatively uncovered so thatetchant and DI water may flow unimpeded. In certain embodiments, lessthan 25% of the wafer perimeter may be covered by the structures;preferably less than 10% of the wafer perimeter may be covered.

The overall angle of the turbulence reducing structure may be about30-90 degrees, preferably 30-60 degrees. The overall angle is the apexangle of a triangle containing the turbulence reducing structure withthe apex pointing in the forward direction of rotation and the baseperpendicular to the wafer. Preferably, a line of the wafer in thehorizontal plane would bisect the triangle. This triangle is shown indotted lines on FIG. 8A as 823 with apex angle 825. The overall angleaffects the drag coefficient and length of the turbulence reducingstructure. A smaller overall angle means longer structure with lowerdrag. A bigger overall angle means shorter structure with bigger drag.

FIGS. 8-11 shows schematic designs for wafer holders that have reduceddrag coefficients. FIG. 8A shows the wafer holder as viewed from thecenter of the chuck looking outward toward the region of the waferperimeter, with the air flow from right to left in consideration of aclockwise rotation. View 8B shows a horizontal view of the front of thewafer holder. A rotating wafer holder, rotating clockwise, would bemoving toward the viewer. Although clockwise rotation is illustratedhere as an example, the present invention is not limited to a clockwiserotation. For example, suitable wafer holders that have reduced dragcoefficients may be designed for a chuck that rotates counter-clockwise,or in both directions.

FIG. 8B shows a wafer holder 800 attached to the end of wafer chuck arm809 at point 807. The wafer may be placed on top of support pins 803,which holders the wafer in place by friction during low rotation. Thealignment member 801 is tapered to align the wafer as it is placed intothe wafer chuck. The alignment pin has a through hole 805, which allowsthe liquid applied to the wafer to drain through the wafer holder. Theclamping member 817, as shown, is the aerodynamically shaped turbulencereducing structure.

The shape of the turbulence reducing structure 817 may be paraboloidhaving a portion of a quadrant removed. The removed portion gives roomfor the wafer to be positioned on top of the supports 803. FIG. 8C showsa view of the wafer holder with a wafer 827 on top of the supports 803.The wafer clearance between the bottom of the wafer and the top of theshelf 813 may be about 0.1-0.5 inch, preferably about 0.3 inch. Theclearance is required to form an escape path for the liquid applied tothe wafer. Without adequate clearance, liquid may accumulate around theshelf. Preferably, the etchant flows down the edge of the wafer and tothe backside unimpeded. Optionally, the structure may include holes atareas above the shelf at element 811 to allow liquid to escape,performing the same function as hole 805. The tip of the structure isshown at the location of arrow 815. In general, the tip may be a pointor may be smooth. Although a pointed tip may be slightly better fordrag, a rounded tip may be preferred because it is safer to handle and apointed tip may be more fragile.

During operation, the alignment member 801 and the clamping member 817rotate in opposite direction at different rotational speeds. At norotation or low rotational speeds, the alignment member 801 stays in thealignment position, ensuring a centered wafer. In this position, thealignment member contacts the edge of wafer when the wafer is placed onthe chuck. More specifically, the chuck arms and their associatedalignment members are positioned so that they contact a wafer beingplaced on the chuck in a manner that causes the wafer to precisely slideinto correct alignment on the wafer chuck. The alignment member in FIG.8 is shown in its inner, alignment position. As the speed increases, thealignment member 801, having a center of gravity higher than a pivot pin819, pivots outward from the wafer due to the centripetal force at aparticular rotational speed. After the alignment member pivots away, thewafer may be etched because the alignment member 801 no longer contactsthe wafer and impedes etchant flow. After the EBR and BSE processes, therotational speed is increased further for the rinsing and drying steps.As the rotational speed increases further, the clamping member 817pivots inward toward the wafer, holding the wafer in position duringhigh-speed rotation such as that used during drying. In someembodiments, the clamping member has a notch that holds the wafer. Theclamping member 817 has a center of gravity lower than that of its pivotpin 821. The clamping force increases with the centripetal force, whichis directly correlated to the rotational speed. One skilled in the artwould be able to design a wafer holder that would align, release thewafer edge, and clamp the wafer at various desired rotational speeds.

FIGS. 9 to 11 shows various other aerodynamic shape types that may beused with the present invention. FIG. 9 shows a turbulence reducingstructure having a pointed end 815 in the shape of a pyramid having aportion removed. As examples, the turbulence reducing structure may bebased on a paraboloid, a cone, or a prismatoid having three or moresides, all with a portion removed, as necessary, to make room for thewafer and clearance. A prismatoid is a polyhedron where all the verticeslie in one of two parallel planes. For example, a pyramid such as thatof FIG. 9 has one vertex in one plane and 4 vertices in another plane. Awedge is also a prismatoid having two vertices in one plane and four inanother.

The clamping member may have turbulence reducing structures on bothends, not just the forward end. FIG. 10 shows an embodiment where thetwo ends are of different shapes. FIG. 11 shows an embodiment where thetwo ends are of the same shape. In FIG. 10, the forward end has apointed tip and the rear end has a round tip. Various other combinationsare possible. For example, a teardrop shape with a rounded forward endand long pointed rear end may have a suitable drag coefficient. FIG. 11shows an example of the “shark fin and snout” shape type. The structurehas an irregular shape having curved faces and lines. The turbulencereducing structure of FIG. 11 is may be described as a wedge or atetrahedron having curved faces with a portion removed. As viewed fromthe wafer, the structure has a curved top and bottom that comes to arounded point (FIG. 11A), with the top resembling a shark fin. As viewfrom the front (FIG. 11B), the structure resembles a shark snout, againwith a portion removed. FIG. 11C shows the same structure in aperspective view.

The present invention is not limited to the embodiments described. Asdiscussed above, any aerodynamic shape having a drag coefficient of 0.5and below, or preferably around 0.2 to 0.3 may be used.

The structure should preferably substantially shield the alignment pinfrom direct airflow while the chuck is spinning. The height of top ofthe structure should be about the top of the alignment member in theengaged position. The structure should also avoid imparting a verticalmomentum on the wafer chuck. In other words, the pressure above andbelow the structure should be balanced; e.g., the drag of the flow aboveand below the structure should be about the same. The wafer chuck shouldnot move vertically while the wafer chuck start to spin or speed up.

Further, the structure preferably has little or no airflow displacementin the horizontal direction, because radial airflow may affect the EBRetchant stream. In other words, the structure should move more air toits top or bottom instead of its sides. A substantial airflow to thesides of the structure may create air currents that can affect the EBRetchant stream. To minimize the horizontal airflow, the structure shouldpreferably have less surface areas toward and away from the wafer thenthe surface areas above and below the wafer. Less surface areas towardand away from the wafer means that less air would be directedhorizontally from the wafer, which may affect the EBR etchant flow asthe structure spins past the stream.

As discussed above, the clamping member rotates 817 to engage the waferduring high-speed rotation. It is envisioned that this rotation would besmall, around 10-45 degrees, preferably around 20 degrees so as toimpart only a very small radial air displacement. At lower speeds, thestructure should not impart any radial displacement so as to minimizeeffect on the EBR etchant stream. At higher speeds when the clampengages the wafer, a small radial air displacement may be toleratedbecause in that regime, the EBR etchant is not flowing.

While this invention has been described in terms of a few preferredembodiments, it should not be limited to the specifics presented above.Many variations on the above-described embodiments may be employed.Note, for example, that other semiconductor processes may also uses arotating wafer chuck that has the same particle and radial precisionconcerns. For example, in CMP, the wafer is also cleaned, rinsed, anddried. The ability to rotate the wafer at very high speeds withoutcausing air turbulence in the chamber may be desirable in manyapplications. Therefore, the invention should be broadly interpretedwith reference to the following claims.

What is claimed is:
 1. A rotatable aerodynamic wafer chuck for a modulefor processing semiconductor wafers in a manner where wafer holders donot substantially affect the stream of fluid and thereby reduces processvariations on a wafer, the rotatable aerodynamic wafer chuck comprising:a. a plurality of chuck arms extending radially about a center of therotatable wafer chuck; and b. a plurality of wafer holders, eachattached to an end of a chuck arm of the plurality of chuck arms andconfigured to engage a semiconductor wafer during rotation of therotatable wafer chuck, wherein each wafer holder of the plurality ofwafer holders comprises: i. a forward end and a rear end along adirection of rotation of the rotatable wafer chuck; and ii. a turbulencereducing structure on the forward end or the rear end or both ends ofeach wafer holder, the turbulence reducing structure comprising a curvedor angled aerodynamically shaped surface, wherein the turbulencereducing structure is oriented and configured such that, when rotatingduring post-electrofill wafer processing using a stream of fluid on atleast a portion of the wafer facing the turbulence reducing structure,the wafer holders do not substantially affect the stream of fluid andthereby reduces process variations on the wafer.
 2. The wafer chuck ofclaim 1, wherein the turbulence reducing structure is shaped like anelliptic paraboloid or a cone, each having a portion of a quadrantremoved.
 3. The wafer chuck of claim 1, wherein the turbulence reducingstructure is shaped like a prismatoid having 3 or more sides.
 4. Thewafer chuck of claim 3, wherein the shape of the turbulence reducingstructure has one or more curved lines.
 5. The wafer chuck of claim 4,wherein the shape of the turbulence reducing structure has one or morecurved sides.
 6. The wafer chuck of claim 1, wherein each wafer holderof the plurality of wafer holders further comprises: a clamping memberrotatable in a perpendicular direction to a wafer surface of thesemiconductor wafer, wherein the clamping member clamps thesemiconductor wafer by constraining the semiconductor wafer at its edge.7. The wafer chuck of claim 6, wherein each wafer holder of theplurality of wafer holders further comprises: a friction support and avertical alignment member; wherein the vertical alignment member and theclamping member are independently rotatable in a perpendicular directionto a wafer surface of the semiconductor wafer; and wherein the clampingmember comprises the forward end and the rear end.
 8. The wafer chuck ofclaim 7, wherein the turbulence reducing structure is configured tosubstantially shield the vertical alignment member from a direct airflowduring chuck rotation.
 9. The wafer chuck of claim 7, wherein at leastone of the vertical alignment members is a cam whose position isdetermined by a centripetal force associated with rotation of the chuck.10. The wafer chuck of claim 7, wherein at least one of the clampingmembers helps hold at least one of the vertical alignment member in analignment position when the rotatable wafer chuck is rotating below afirst certain speed and releases the vertical alignment member into anon-alignment position when the rotatable wafer chuck is rotating at orabove a second certain speed.
 11. The wafer chuck of claim 6, wherein atleast one of the clamping members is a cam whose position is determinedby centripetal force associated with rotation of the chuck.
 12. Thewafer chuck of claim 1, wherein the turbulence reducing structure has adrag coefficient of about 0.5 or less.
 13. The wafer chuck of claim 1,wherein the turbulence reducing structure has a drag coefficient ofabout 0.2 to 0.3.
 14. The wafer chuck of claim 1, wherein an overallangle of the turbulence reducing structure is about 30-60 degrees; andwherein the overall angle is an apex angle of a triangle containing theturbulence reducing structure with the apex pointing in the forwarddirection of rotation and a base perpendicular to the wafer.
 15. Thewafer chuck of claim 1, wherein the turbulence reducing structure doesnot cover more than about 10% of the wafer perimeter when the stream offluid is delivered.
 16. The wafer chuck of claim 1, wherein the moduleis configured to etch metal from the semiconductor wafer.
 17. Arotatable aerodynamic wafer chuck for a module for processingsemiconductor wafers in a manner where wafer holders do notsubstantially affect the stream of fluid and thereby reduces processvariations on a wafer, the rotatable aerodynamic wafer chuck comprising:a. a plurality of chuck arms extending radially about a center of therotatable wafer chuck; and b. a plurality of wafer holders, eachattached to an end of a chuck arm of the plurality of chuck arms andconfigured to engage a semiconductor wafer during rotation of therotatable wafer chuck, wherein each wafer holder of the plurality ofwafer holders comprises a turbulence reducing structure comprising acurved or angled aerodynamically shaped surface, wherein the turbulencereducing structure is oriented and configured such that, when rotatingduring post-electrofill wafer processing using a stream of fluid on atleast a portion of the wafer facing the turbulence reducing structure,the wafer holders do not substantially affect the stream of fluid andthereby reduces process variations on the wafer.
 18. A module forprocessing semiconductor wafers, the module comprising: a. a chamberhaving chamber walls; b. a fluid dispensing mechanism configured todeliver fluid to a wafer surface, and c. a rotatable wafer chuckcomprising: i. a plurality of chuck arms extending radially about acenter of the rotatable wafer chuck; and ii. a plurality of waferholders, each attached to an end of a chuck arm of the plurality ofchuck arms and configured to engage a semiconductor wafer duringrotation of the rotatable wafer chuck, wherein each wafer holder of theplurality of wafer holders comprises a turbulence reducing structurecomprising a curved or angled aerodynamically shaped surface, whereinthe turbulence reducing structure is oriented and configured such that,when rotating during post-electrofill wafer processing using a stream ofthe fluid on at least a portion of the wafer facing the turbulencereducing structure, the wafer holders substantially prevent droplets onthe chamber walls from being entrained, and wherein the turbulencereducing structure does not cover more than about 10% of the waferperimeter when the stream of fluid is delivered.
 19. A module forprocessing semiconductor wafers, the module comprising: a. a chamberhaving chamber walls; b. a fluid dispensing mechanism configured todeliver fluid to a wafer surface, and c. a rotatable wafer chuckcomprising: i. a plurality of chuck arms extending radially about acenter of the rotatable wafer chuck; and ii. a plurality of waferholders, each attached to an end of a chuck arm of the plurality ofchuck arms and configured to engage a semiconductor wafer duringrotation of the rotatable wafer chuck, wherein each wafer holder of theplurality of wafer holders comprises a turbulence reducing structure,wherein the turbulence reducing structure has less surface area towardand away from the wafer and more surface area above and below the wafer,and wherein the turbulence reducing structure is oriented and configuredsuch that, when rotating during post-electrofill wafer processing usinga stream of fluid on at least a portion of the wafer facing theturbulence reducing structure, the wafer holders do not substantiallyaffect the stream of fluid and thereby reduces process variations on thewafer.